1. Field of the Invention
This invention relates generally to a non-contact voltage detector. More specifically, this invention relates to a non-contact voltage detector having a self-test circuit.
2. Description of Related Art
Non-contact voltage detectors are used in a variety of situations when it is necessary to determine if a line or cable is “live” or “hot” because it is connected to a source of voltage, typically an AC voltage. One such situation is when an electrical worker plans to work on the line and wants to ensure that the line is not energized before the worker touches it and risks electrical injury. Non-contact voltage detectors are well suited to this task because they allow the worker to determine if the line is energized without touching the line so that the worker can obtain the information needed without risking injury.
Conventional non-contact voltage detectors devices work by using an antenna or pickup plate that is metallic to conduct a small capacitively-coupled current signal when it is placed into an alternating electrical field. The antenna is usually covered by plastic or other insulating material, thin and elongated, and located near the tip of the device so that the antenna can fit into tight spaces where the cable may be located. The plastic helps to protect the antenna from being broken and also protects the user from inadvertent contact with live voltage. This portion is often referred to as the probe.
After the worker has identified a particular cable, the worker places the probe next to or in direct contact with the cable. If the cable is energized, the metallic antenna is in the presence of an alternating electrical field. This causes the antenna to induce a capacitively-coupled current signal which is conveyed to a detection circuit. In response, the detection circuit triggers a beeper to sound, an LED to flash, or some other signaling device to activate to alert the user that the cable is energized. If the cable is not energized, no current signal is induced by the antenna and the worker is not alerted by the beeper, LED, or other device. The worker then assumes that it is safe to work on the cable.
The detection circuit, beeper and LED are usually battery powered. It is critical that the batteries provide sufficient power to the detection circuit and signaling devices. If not, the non-contact voltage detector will fail to provide a warning signal to the worker even if the cable is energized. For this reason, some conventional non-contact voltage detectors employ a battery verification self-test circuit which is used to indicate to the worker that the batteries have enough power. For example, when the batteries are sufficiently charged, the battery verification self-test circuit may continuously illuminate an LED, emit a beep sound or flash an LED when the unit is turned on, or periodically flash an LED for the entire time the unit is powered on.
However, while these implementations may safeguard against the possibility of a worker trusting a non-contact voltage detector having batteries that are not providing sufficient power to the detection circuit, they do not verify the integrity of the signal path of the detection circuit and/or the antenna.
Therefore, there is a need for a non-contact voltage detector having a self-test circuit that is configured to verify that a signal sent to the detection circuit can be received and detected by the detection circuit. Additionally, there is a need for a non-contact voltage detector having a self-test circuit that is configured to verify that a signal received by a portion of the antenna will travel through the antenna and to the detection circuit where it will be received and detected.